Inset panel fastener and method of using

ABSTRACT

A fastener element (70, 104, 120) is injection molded from an engineering resin of a type that will bond to potting resin (68) used for securing the fastener element (70, 104, 120) within the cavity (24) formed within a panel (12) of composite material. The fastener element (70, 104, 120) is constructed to include disks (80, 82, 106, 122, 124) spaced apart along a stem portion (76, 108, 128), between end portions (72, 74, 130, 136). Flats (90, 92, 93, 95, 114, 116, 118, 138, 140, 142, 144, 146, 148) are formed on opposite sides of the fastener element (70, 104, 120), below fill and vent openings (94, 96, 110, 112, 132, 134). This construction results in resin travel down to the bottom of the cavity (24) and around the fastener element (70, 104, 120) and a filling of the cavity (124) from the bottom up, so that the presence of resin at the bent opening (96, 112, 134) will indicate that the cavity (24) has been filled with resin and does not include an airspace or void.

TECHNICAL FIELD

This invention relates to inset fastener elements for sandwichstructures. More particularly, it relates to the provision of animproved inset fastener element and an installation method for use withsandwich panels constructed from composite materials.

BACKGROUND ART

A sandwich panel combines high strength, inherent rigidity and minimumweight. For this reason it is used extensively in the aircraft industry.Of primary importance to successful utilization of a sandwich panel isthe method of transmitting loads into and out of it. By their verynature, sandwich panels cannot carry concentrated unit loading. Fastenerinserts have been developed for use with these panels. The presentinvention relates to improvements in molded-in type fastener inserts.

A prior art insert and a prior method of installing such insert havebeen illustrated and will be hereinafter described. Such prior artfastener element is typical of the fastener elements which haveheretofore been used with both sandwich panels constructed of metal andsandwich panels constructed from composite material. A problem with theprior art insert is that it is constructed from a metal and the resinused for securing it within the sandwich panel does not bond to themetal. As a result, the fastener element is only held in place by amechanical interlock which is formed between the resin and flangeportions of the fastener element. Owing to the construction of the priorart fastener elements, often times air spaces or voids are formed in thepotting resin, producing a weakened connection.

The principal object of the present invention is to provide an improvedfastener element and a method of installing such fastener element whichwill result in a stronger connection of the fastener element to thesandwich panel and an elimination or reduction in size of airspaces orvoids in the potting resin.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, a fastener element is providedwhich is injection molded from an engineering resin of a type that willmake a cohesive bond with the potting resin. Such fastener elementcomprises an outer end disk sized to be snugly received within a skinopening. A stem projects inwardly from the outer end disk, into a cavityformed in the panel which has a diameter larger than the diameter of theskin opening. The stem is substantially smaller in diameter than theouter end disk. The fastener element includes flow control disksextending about the stem. Flow passageways are defined by and betweenthe disks means and between the outer end disk and the disks. The flowcontrol disk are formed to include flat surfaces on diametricallyopposed sides of the fastener element. The flat surfaces are located inplanes which are spaced radially inwardly from the periphery of theouter end disk. On one side of the fastener element a resin receivingfill opening is formed in a peripheral portion of the end disk,generally in line with a region in a formed cavity between the flats andthe adjoining side of the cavity. A vent opening is formed in a secondperipheral portion of the end disk that is spaced diametrically acrossthe fastener element from the fill opening. The vent opening is inalignment with a second cavity region formed between flats on the ventopening side of the fastener element and an adjoining sidewall of thecavity.

The fastener element is set into the cavity with the end disksubstantially flush with the skin of the panel. Potting resin isintroduced into the fill opening. Owing to the construction of thefastener element, the potting resin first substantially fills the regionof the cavity which is inwardly of the fill opening and laterallybetween the flats on the fill opening side of the fastener element andthe adjoining sidewall of the cavity. The resin then flows around thefastener element and through the avenues between the disks. Theseavenues cause the potting resin to flow into all parts of the cavityregion defined inwardly of the vent opening. The entire cavity isessentially full of resin before resin shows at the vent opening.

The flats help define cavity regions through which the potting resinwill readily flow, and provided a substantial area to resist turning ofthe fastener element relative to the potting material and the sandwichpanel. The spaces between the disks define avenues for evenlydistributing the potting resin around the fastener element, and fordirecting it into all required portions of the cavity region inwardly ofthe vent opening, to either prevent or minimize the formation ofairspaces or voids in the potting resin. The disks and flow avenuesfurther provide a superior mechanical interlock between the fastenerelement and the potting resin, functioning together with the bondingthat occurs between the potting resin and the fastener element resin,for securing the fastener element in place and enabling it to withstandloads of a magnitude which would pull the prior art fasteners out fromthe sandwich panel.

These and other features, advantages and characteristics of theinvention will be apparent from the various embodiments of the inventionwhich are illustrated and described below in connection with thedrawing.

BRIEF DESCRIPTION OF THE DRAWING

Like reference numerals are used to designate like parts throughout theseveral views of the drawing, and:

FIG. 1 is a fragmentary isometric view of a honeycomb panel, with aforeground quarter section removed, for clarity of illustration, suchview showing a blind opening that has been drilled down into the panel;

FIG. 2 is a fragmentary sectional view, showing a cutter element beingwithdrawn from the opening;

FIG. 3 is a view like FIG. 2, showing the use of a tool for routing outcore material for the purpose of forming an enlarged cavity inwardly ofthe entrance portion of the opening;

FIG. 4 is an exploded side elevational view of a prior art fastenerelement and an istallation tool with the fastener element shown insections;

FIG. 5 is an exploded isometric view of the prior art fastener elementand installation tool;

FIG. 6 is an assembled side elevational view of the prior art fastenerelement and installation tool;

FIG. 7 is an isometric view of a fragment of a panel, showing theinstallation tool being used for installing the prior art fastenerelement;

FIG. 8 is a side elevational view of the assembly shown by FIG. 7, withparts shown in section;

FIG. 9 is a view like FIG. 7, showing a resin material being introducedinto the enlarged cavity;

FIG. 10 is a view like FIG. 8, showing the resin being introduced intothe enlarged cavity;

FIG. 11 is a view like FIGS. 7 and 9, following completion of theinstallation of the fastener elements;

FIG. 12 is a view like FIGS. 8 and 10, following installation of thefastener element;

FIG. 13 is an isometric view of a first embodiment of fastener elementconstructed according to the present invention;

FIG. 14 is a top plan view of the fastener element shown by FIG. 13;

FIG. 15 is a side elevational view of the fastener elements shown byFIGS. 13 and 14;

FIG. 16 is a second side elevational view of the fastener element shownby FIGS. 13-15, showing a side spaced 90° from the side shown by FIG.15;

FIG. 17 is a view like FIG. 10, but showing the fastener element ofFIGS. 13-16;

FIG. 18 is a view like FIG. 12, but showing the fastener elements ofFIGS. 13-17;

FIG. 19 is a view like FIG. 11, but showing the fastener element ofFIGS. 13-18;

FIG. 20 is an isometric view of a second embodiment of fastenerconstructed according to the present invention, such fastener elementhaving a single helical rib;

FIG. 21 is a side elvational view of a third embodiment of fastenerelement;

FIG. 22 is a side elevational view of the embodiment shown by FIG. 21,looking towards a side that is spaced 90° from the side shown by FIG.21;

FIG. 23 is a side elevational view of the helical rib embodiment shownby FIG. 20, looking towards one of the sides which includes flats.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1-12 illustrate a prior art fastener element 10 and a prior artmethod of installing such fastener element 10. For example purposes, athreaded blind opening fastener has been illustrated, of a typemanufactured by the Shur-Lok Corporation of 1300 East Normandy Pl.,Santa Ana, Calif. 92711, series SL601. It comes in several sizes and isconstructed from aluminum, steel or CRES. The panel is known as a"sandwich panel." The present invention is concerned with installing afastener element into a sandwich panel 12 constructed from compositematerials. It comprises opposite surface forming skins 14, 16constructed from a suitable engineering plastic (e.g. fiberglass) and ahoneycomb core 18 constructed from a plastic or treated paper (e.g.Nomex). As is well-known, the skin layers 14, 16 are resin bonded to thecore material 18.

It is known to mold an aluminum fastener element 10 into a panel 12constructed from composite materials. This known technique will now bedescribed, with reference to FIGS. 1-12.

A cutting bit 20 is used to cut an opening through first one skin 14 andthen most of the core 18. Of course, if a through type fastener elementis used, the opening will be cut through the entire panel 12.

In FIGS. 1 and 2, the opening through the skin member 14 has beendesignated 22. The opening or cavity formed in the core 18 has beendesignated 24. Following formation of openings 22, 24, a reaming tool 26is used for enlarging the opening 24. Tool 26 may comprise a rotarychuck 28 and a reamer 30 secured in and rotated by the chuck 28. Asshown by FIG. 3, the tool 26 is used to enlarge the opening 24 until ithas a diameter substantially larger than the diameter of opening 22.

Fastener element 10 has an outer end portion 32 which is disk shaped andis substantially equal in diameter to the opening 22. Fastener element10 also comprises a stem 34 which is substantially smaller in diameterthan outer end portion 32 and further includes an inner end portion 36.End portion 36 is substantially equal in diameter to end portion 32 butis formed to include flats 38, 40 on diametrically opposite sides. Theseflats 38, 40 are referred to as antirotation flats. A girth groove 42 isformed in the stem 34 closely adjacent end portion 32. An internallythreaded blind socket 44 extends axially into the insert 10. Twoopenings 46, 48 are formed in the end portion 32, outwardly of the stem34. Opening 46 is a "fill" opening and opening 48 is a "vent" opening.

The installation of the fastener element 10 will now be described. Apositioning tab 50 is provided which includes a main body portion thatis larger than the opening 22. Tab 50 includes openings 52, 54 whichcorrespond in size and spacing to the openings 46, 48. Tab 50 isprovided with a handle 56 and an adhesive layer 58 on one side. The tab50 comes with a paper backing for protecting the adhesive layer 58. Thispaper backing is removed and the tab 50 is positioned on a guide tool60. Guide tool 60 includes a pair of projections 62, 64 sized and spacedto extend through the openings 52, 54. The fastener element 10 is theninstalled on the tool 60, with the projections 62, 64 extending throughthe openings 46, 48. The fastener element 10 is pressed against theadhesive layer 58. Next, the fastener element 10 is positioned withinthe cavity 24 (FIGS. 7 and 8). Downward pressure is applied on the tab50 for the purpose of creating an adhesive bond between the adhesivelayer 58 and the upper surface portion of skin 14 immediatelysurrounding opening 22. Then, the tool 60 is removed, leaving tab 50connected to skin 14 and fastener element 10 connected to tab 50. Next,the nozzle 66 of a sealant gun is brought into registry with openings52, 46. An epoxy potting material 68 is injected through these openingsinto the cavity 24. Air within cavity 24 is displaced out throughopenings 48, 54 as epoxy is introduced into the cavity 24. In theory,the appearance of epoxy 68 at the openings 48, 54 tells the workman thatthe cavity 24 has been filled. In actual practice, owing to theconstruction of the fastener element 10, epoxy 68 may flow about theupper portion of stem 34, from fill openings 52, 48 to vent openings 48,54, before the lower portion of cavity 24, on the side thereof oppositethe fill openings 52, 46, has received epoxy 68. And/or air bubbles willform around the fastener and hold the resin away from the fastener.Voids of this type are shown in FIGS. 10 and 12. This results in aninadequate anchoring of the fastening element 10 within the cavity 24,as will be described some more below.

The workman is trained to discontinue ejecting epoxy 68 as soon as hesees epoxy at the vent openings 54, 48. He removes the nozzle 66 andalso removes the tab 50. Adhesive 58 is a pressure type adhesive whichwill adequately connect the tab 50 to the fastener element 10 and theskin member 14, during the installation process, but will readilyseparate from the skin member 14 and end portion 32 by the workmanpulling on handle 56 to peel the tab 50 away from the members 14, 32.

As previously mentioned, the fastener insert may be a through typeinsert. In that case a second opening is formed in the skin member 16 toreceive a second end portion which is snugly received within suchopening. In other respects, the installation technique is the same ashas been illustrated and described. An enlarged cavity is formed in thecore material 18 and epoxy 68 is introduced into the cavity, between itssidewall and the stem portion of the fastener element.

The prior art practice of installing an insert in a sandwich panel ofcomposite material has always involved the use of a metal insert. Theillustrated insert 10 is a typical configuration. In other words, theinsert has a generally spool configuration, and includes enlarged endsand a smaller diameter stem between the ends. Structural tests conductedwith respect to these fasteners has indicated that the fastener elementis held in place only by a mechanical interlock. In other words, theepoxy that enters into the annular zone surrounding the stem 34, betweenthe end members 32, 36, provides a mechanical interlock for resisting apulling out of the fastener element 10. The tests have indicated thatthis type of structural interlock is often times ineffective,particularly in those cases where an airspace or void has resulted inthe cavity 24.

In accordance with the present invention, the fastener element isconstructed from an engineering plastic of a type to which the resin 68will cohesively bond. The resin 68 becomes cohesively bonded to the skinmembers 14, 16 and may also be cohesively bonded to the core material18. The construction of the fastener insert from a resin compatible toresin 68 results in a true molding-in of the fastener element. Further,the fastener elements of the present invention are constructed in amanner resulting in an elimination or substantial reduction in size ofany void region in the cavity 24. The fastener elements are alsoconstructed to provide substantial surfaces fOr resisting rotation ofthe insert relative to the panel 12.

Referring to FIGS. 13-19, the fastener insert 70 shown by these figuresis molded from an engineering resin. By way of example, a poly(amide-imide) molding resin may be used of a type manufactured by AmocoChemicals Corporation of 200 East Randolph Dr., Chicago, Ill. 60601.This particular resin is a reaction product of trimellitic nahydride andaromatic diamines. It is called amide-imide because the polymer chaincomprises amide linkages alternating with imide linkages. Themanufacturer claims that (1) the combination of aromatic groups andimide linkages are responsible for the polymer's exceptional thermalstability and (2) the amide groups impart flexibility and elongation,which results in an engineering plastic with exceptional toughness. Thisparticular resin is sold under the trademark TORLON, which is aregistered trademark of AMOCO Chemical Corporation.

Referring to FIGS. 13-16, fastener element 70 has a pair of end portions72, 74, interconnected by a stem 76. Fastener element 70 is a blindthreaded type fastener element. Its inner end portion 74 includesantirotation flats 78 on diametrically opposite sides. Flow controldisks are provided on the stem 76, between the end portions 72, 74, andspaces flow passageways are formed between adjacent flow control disksand between the end flow control disks and the end portions 72, 74. Inthe embodiments shown by FIGS. 13-16, the flow control disks comprisecircular ribs 80, 82 which extend about the girth of stem 76. The flowpassageways are girth grooves 84, 86, 88. Disks 80, 82 are provided withflats 90, 92 on the same side of element 70 as flat 78, and flats 93, 95on the same side of element 70 as flat 79. A fill opening 94 is providedin a peripheral part of end portion 72, generally in alignment with theside region of the element 70 laterally outwardly of the flats 90, 92,78. Vent opening 96 is formed in end portion 72, diametrically oppositefill opening 94. Vent opening is in alignment with the side region ofelement 70 that is laterally outwardly of the flats 93, 95, 79. Aninternally threaded aligned central blind opening 98 enters the element70 through end portion 72, and extends axially into the element 70.

As shown by FIGS. 13-16, the flats 90, 92, 78 and 93, 95, 79 may lie onchords which extend at a tangent to the stem 76.

As shown by FIGS. 15 and 16, a portion 100 of the stem 76 may extendbelow end member 74.

A positioning tab 50 and installation tool 60 are used for positioningfastener element 70 relative to the cavity 24 in panel 12 as previouslydescribed. Epoxy 68 is introduced in a nozzle 102 into the fill opening94.

Owing to the construction of insert 70, the first resin 68 to enterthrough fill opening 94 will flow into a side region of cavity 24outwardly bounding the flats 90, 92, 78. The surfaces of the end member72, 74 and the disks 80, 82 will retard flow of the resin 68 around tothe opposite side of insert 70 until a substantial amount of the resin68 is accumulated in the introduction region. The flow passageways orspaces and the disks 80, 82 and end member 74 influence the flow aroundthe fastener element 70. Enough pressure is developed in theintroduction region to cause the resin 68 to flow faster around thelower portion of the fastener element 70 than it does about the upperportion of fastener element 70. As a result, this allows the ventopening 96 to function effectively and results in resin filling theregion that outwardly bounds the flats 93, 95, 79 from the bottom up, sothat resin does not appear at the vent opening 96 until the cavity 24 isin fact substantially full of resin.

The positioning of the flats 90, 92, 78 and 93, 95, 79 contributes tothe flow control of the resin. They also function to provide asubstantial amount of resistance to rotation. Prior art fastenerelements include a single pair of antirotation flats at the lower end ofthe fastener element. The fastener element of the present inventionincludes four additional flats 90, 92, 93, 95, and further provides flowcontrol of the resin. As stated above, the fastener element 70 isconstructed from a resin material to which the resin 68 actually bonds.Tests have shown that the cohesive attraction between the resin and thefastener element 70 by itself resists the formation of air pockets orvoids next to the fastener elements, 70. The presence of disks 80, 82enhances the mechanical interlock between the fastener element 70 andthe resin 68. This enhanced mechanical interlock and the bond betweenthe materials provides a much improved connection of the fastenerelement 70 to the panel 12. In actual tests, the fastener element 70 waspulled upon until a failure occurred. The failure was in the panel 12surrounding the body of resin 68, and not a mere mechanical interlockfailure immediately surrounding the fastener element 70. The forcerequired to cause such failure was considerably larger than the forcerequired to cause the failure of the connection of the prior artfastener 10 to the panel 12. Also, the test disclosed that the voidforming problem was essentially eliminated.

FIG. 20 shows a modified construction of the insert. The insert 104shown by this figure comprises rib means in the form of a continuoushelical rib 106, the turns of which form the function of disks 80, 82and end portion 74, in the embodiment shown by FIGS. 13-19. A helicalgroove or space 108 is formed between the turns of the helical rib 106.This space 108 performs the function of the spaces between the disks 80,82, between disk 80 and end member 72 and between disk 82 and end member74. The turns of the helical rib 106 is formed to include flats in theregions below a fill opening 110 and below a vent opening 112. The flats114, 116, 118 below the fill opening 110 are shown. Similar flats areprovided on the opposite side of the fastener element 104, below thevent opening 112. Fastener element 104 is also a blind type fastenerelement. It includes an axially extending, internally threaded blindopening 118, for receiving the threaded portion of a bolt (not shown).

FIGS. 21, 22 show a fastener element 120 which is like fastener element,70 except that the under surface portions of the disks 122, 124 and theundersurface portion of the end portion 126 are slanted. Specifically,this surface rises at it extends outwardly from the stem 128. Endportion 130 is like end portion 72 of fastener element 70. It includes apair of diametrically opposed openings 132, 134, one of which is a fillopening and the other of which is a vent opening. It also includes anaxially extending blind opening 136 for receiving the threaded portionof bolt. Flats 138, 140, 142 are formed on the disks 122, 124 and endmember 126, below opening 132. In similar fashion, on the opposite sideof the fastener element 120, flats 144, 146, 148 are formed on the disks138, 140 and on the end portion 142. This embodiment is installed in thesame manner as the embodiment shown by FIGS. 13-19. The sloping surfacesfunction to help direct the resin to flow in the avenues below disks122, 124 and below end member 126 more readily than in the groovebetween end member 130 and disk 122. The outwardly and upwardly slopinglower surfaces of the disks 122, 124 and end member 126 increase thesizes of the avenues for resin flow which are defined below the elements122, 124, 126. FIG. 21 clearly shows that the forwardly and outwardlysloping lower surfaces of the disks 122, 124 makes the avenues below thedisks 122, 124 larger in cross section than the avenue formed betweendisk 122 and end member 130. This construction and the head pressure onthe resin being introduced into the cavity will encourage resin flowaround the bottom and center portions of the fastener element 120 thanabout the upper portion of fastener element 120. This results in asubstantial filling of the cavity before resin shows at the vent opening134.

Other changes in the construction of the insert can be made withoutdeparting from the basic principles of the invention. As earliermentioned, the principles of the invention can be applied to a throughtype fastener element, having either a through clearance or a throughthreaded type opening. The depth of the spacing between the end flangeand disks may vary, so that the space increases in depth somewhat fromthe top down.

The illustrated embodiments are presented for example purposes. Thedescription of these embodiments is not to define or limit the scope ofthe invention. Rather, the scope of the invention is to be determinedsolely by an interpretation of the claims, applying the establishedrules of patent claim interpretation, including the doctrine ofequivalents.

What is claimed is:
 1. A method of embedding a fastener element into apanel of composite material, comprising:providing a skin opening in thepanel that is of a first diameter; providing a cavity within said panel,inwardly of the skin opening, which is larger in diameter than the skinopening; providing a fastener element made from a resin material;providing the fastener element with an outer end disk sized to be snuglyreceived within the skin opening and a circular stem projecting inwardlyfrom the outer end disk which is substantially smaller in diameter thanthe outer end disk; providing the fastener element with a plurality ofaxially spaced apart flow control disks on the stem, defining flowpassageways around the stem between the flow control disks and betweenthe outer end disk and an adjacent flow control disk, each of which hasan axial dimension substantially smaller than the stem diameter betweenthe flow control disks; providing the flow control disks with flat chordsurfaces on diametrically opposite sides of the fastener element, withthe flat chord surfaces on one side of the fastener element beingparallel to the flat chord surfaces on the opposite side of the fastenerelement; providing the regions of the flow control disks which areangularly between the chord flats with diameters substantially equal tothe diameter of the outer end disk; positioning the fastener elementwith its flow control disks, its flow passageways and the stem withinthe cavity and with its outer end disk within the skin opening;providing the outer end disk with a peripheral fill opening which is inaxial alignment with a space formed in the cavity radially outwardly ofthe chord flats on a first side of the fastener element; providing theouter end disk with a peripheral vent opening diametrically across thefastener element from the fill opening, and in axial alignment with aregion in the cavity located radially outwardly from the chord flats onthat side of the fastener element; introducing into the fill opening apotting resin of a type which will make a cohesive bond with thefastener element resin material, and continuing introduction of thepotting resin until such potting resin flows first axially through thecavity region in general alignment with the fill opening, and thenaround the fastener element, including around the stem and through theflow passageways, to the region of the cavity in axial alignment withthe vent opening, and then axially through said region to the ventopening.
 2. A method according to claim 1, comprising constructing thefastener element from a poly(amide-imide) molding resin and for thepotting resin using an epoxy resin.
 3. A method according to claim 1,comprising providing the flow control disks with inner side surfaceswhich slope axially outwardly as they extend radially outwardly, so thatthe passageways between the flow control disks are larger incross-sectional dimension than the passageway defined between the enddisk and the adjoining flow control disk.
 4. A method of embedding afastener element into a panel of composite material,comprising:providing a skin opening in the panel that is of a firstdiameter; providing a cavity within said panel, inwardly of the skinopening, which is larger in diameter than the skin opening; providing afastener element made from a resin material; providing the fastenerelement with an outer end disk sized to be snugly received within theskin opening and a circular stem projecting inwardly from the outer enddisk which is substantially smaller in diameter than the outer end disk;providing the fastener element with a helical flow control rib on thestem, defining flow passageways around the stem between turns of theflow control rib, each of which has an axial dimension substantiallysmaller than the stem diameter; providing the flow control rib with flatchord surfaces on diametrically opposite sides of the fastener element,with the flat chord surfaces on one side of the fastener element beingparallel to the flat chord surfaces on the opposite side of the fastenerelement; providing the regions of the flow control rib which areangularly between the chord flats with diameters substantially equal tothe diameter of the outer end disk; positioning the fastener elementwith its flow control rib, its flow passageways and the stem within thecavity and with its outer end disk within the skin opening; providingthe outer end disk with a peripheral fill opening which is in axialalignment with a space formed in the cavity radially outwardly of thechord flats on a first side of the fastener element; providing the outerend disk with a peripheral vent opening diametrically across thefastener element from the fill opening, and in axial alignment with aregion in the cavity located radially outwardly from the chord flats onthat side of the fastener element; introducing into the fill opening apotting resin of a type which will make a cohesive bond with thefastener element resin material, and continuing introduction of thepotting resin until such potting resin flows first axially through thecavity region in general alignment with the fill opening, and thenaround the fastener element, including around the stem and through theflow passageways, to the region of the cavity in axial alignment withthe vent opening, and then axially through said region to the ventopening.